1. Field of the Invention
The present invention relates to a piston type variable displacement compressor. More specifically, this invention relates to a piston type variable displacement compressor capable of efficiently adjusting the pressure in the crank chamber.
2. Description of the Related Art
In general, compressors are mounted in vehicles to supply compressed refrigerant gas to the vehicle's air conditioning system. To maintain air temperature inside the vehicle at a level comfortable for the vehicle's passengers, it is important to utilize a compressor whose displacement is controllable. One known compressor of this type controls the inclined angle of a swash plate, tiltably supported on a drive shaft, based on the difference between the pressure in a crank chamber and the suction pressure, and converts the rotational motion of the swash plate to the reciprocal linear motion of each piston.
A conventional piston type compressor disclosed in U.S. Pat. No. 5,173,032 uses no electromagnetic clutch for the transmission and blocking of power between an external driving source and the drive shaft of the compressor. The external driving source is coupled directly to the drive shaft.
The clutchless structure with the driving source coupled directly to the drive shaft can eliminate shocks which otherwise may be produced by the ON/OFF action of such a clutch. When such a compressor is mounted in a vehicle, passenger comfort is improved. The clutchless structure can also contribute to reducing the overall weight and cost of the cooling system.
In such a clutchless system, the compressor runs even when no cooling is needed. With such compressors, it is important that when cooling is not needed, the discharge displacement be reduced as much as possible to prevent the evaporator from frosting. When no cooling is needed or there is a change of frosting, the circulation of the refrigerant gas through the compressor and its external refrigeration circuit should be stopped. The compressor described in the aforementioned U.S. patent is designed to block the flow of gas into the suction chamber from the external refrigeration circuit by the use of an electromagnetic valve.
In the compressor described above, when the circulation of the gas from the external refrigeration circuit to the suction chamber is blocked, the pressure in the suction chamber drops and the control valve responsive to that pressure opens fully. The full opening of the control valve allows the gas in the discharge chamber to flow into the crank chamber, which in turn raises the pressure inside the crank chamber. The gas in the crank chamber is then to the suction chamber. Accordingly, a short circulation path is formed which passes through the cylinder bores, the discharge chamber, the crank chamber, the suction chamber and back to the cylinder bores.
When the pressure in the suction chamber falls, the suction pressure in the cylinder bores falls, too, thus increasing the difference between the pressure in the crank chamber and the pressure in the cylinder bores. This pressure differential in turn minimizes the inclination of the swash plate which reciprocates the pistons. As a result, the discharge displacement is minimized. At this time, the driving torque needed by the compressor is minimized, thus reducing power loss as much as possible.
When the gas flow to the suction chamber from the external refrigeration circuit starts again, the pressure in the suction chamber rises, and then the control valve closes. This inhibits the gas flow into the crank chamber from the discharge chamber, lowering the pressure in the crank chamber. As in the pressure in the suction chamber rises, the suction pressure in the cylinder bores rises, too. The difference between the pressure in the crank chamber and the pressure in the cylinder bores therefore becomes smaller, and the inclined angle of the swash plate reaches a maximum, maximizing the discharge displacement. At this time, the torque needed to drive the compressor becomes maximum.
The aforementioned electromagnetic valve performs a simple ON/OFF action to instantaneously stop or restart the gas flow from the external refrigerant circuit into the suction chamber. Accordingly, the amount of gas supplied into the cylinder bores from the suction chamber decreases or increases drastically. This rapid change in the amount of gas flowing into the cylinder bores causes an abrupt change in the discharge displacement, rapidly decreasing or increasing the discharge pressure. Consequently, the driving torque needed to drive the compressor greatly changes over a short period of time.
When this type of compressor is mounted in a vehicle, one should consider the problem of engine stall. Engine stalling is caused by the torque necessary for driving accessories, such as an alternator or an oil pump for a powered steering mechanism, and the increasing torque needed to drive the compressor. To eliminate the causes of engine stall, an idle speed controller (hereinafter referred to as ISC) is used. The ISC adjusts the amount of air supplied to the engine during idle to keep the engine speed (hereinafter called idling engine speed) at a target value.
The ISC performs feedback control of an actuator, which adjusts the amount of air supplied to the engine to hold the actual engine speed to a target value. During idle, however, when the load of the compressor or the like drastically increases, the engine speed rapidly decreases. In this case, the feedback control of the ISC cannot follow the rapid change in cycle speed, causing the engine to stall. The compressor disclosed in the above-described U.S. patent does not suggest how to avoid engine stall caused by the increased torque needed to drive the compressor.
The target value of the idling engine speed for the case where the load of the compressor or the like is applied to the engine may be set higher than the target value where the load of the compressor or the like is not applied to the engine. With this scheme, even when the load of the compressor or the like on the engine increases drastically at the idling time, causing the rapid decrease in the engine speed, the engine speed is increased, thus avoiding engine stall. If the speed of the idling engine is increased, however, the fuel consumption of the vehicle is increased.